Polyester resins have widespread utility in a variety of applications, such as films, thermoforming, and blow molding. Additionally, polyesters have widespread acceptance and use in today's consumer products. Polyesters, such as PET and polyethylene naphthalate (PEN), are used in disposable products such as food and beverage contains, and especially carbonated drink containers, photographic film, magnetic tape, and the like.
In an effort to conserve resources, several methods have been disclosed in the literature for recycling polyesters. Some methods involve depolymerization, i.e., breaking the ester bond and reducing the polymer to its monomer components. Others processes blend virgin polymer with post-consumer waste materials. These latter processes tend to be simpler and the equipment is less expensive. However, these simpler processes are not without their problems. These processes cannot remove many of the variable constituents, such as colorants and catalyst metals, present in post-consumer polyesters.
Depolymerization of post-consumer polyester into its monomeric components offers more promise since the monomers can in some cases be purified by techniques well known in the art such as distillation, crystallization and filtration. The pure recycle monomers can subsequently be fed to a polyester production process. The cost of the purification steps, however, can make the recycle monomers more expensive than virgin raw materials.
Various methods have been disclosed in the literature for depolymerization of post-consumer polyesters into their component monomers, such as ethylene glycol and terephthalic acid, naphthalic acid or their derivatives, so they could be reused.
For example, U.S. Pat. No. 3,037,050 discloses the recovery of terephthalate acid dimethyl ester by treating polyethylene terephthalate in the form of bulky or lumpy solid masses with super-heated methanol vapor in the presence of any suitable transesterification catalyst substantially at atmospheric pressure.
U.S. Pat. No. 3,321,510 discloses a process for decomposing polyethylene terephthalate by treating with steam at a temperature of from about 200.degree. C. to 450.degree. C. The steam-treated polyethylene terephthalate is then reduced from a brittle solid product to a powder having a mean particles size of from about 0.0005 to 0.002 millimeters, after which the fine powder is atomized with a gaseous substance including inert gas and methanol vapor to from an aerosol. The aerosol is conducted through a reaction zone at a temperature of 250.degree. C. to 300.degree. C. in the presence of excess methanol vapors.
U.S. Pat. No. 3,776,945 discloses a process of depolymerizing polyethylene terephthalate waste to obtain dimethyl terephthalate and ethylene glycol. The waste is subdivided into dimensions between 4 and 35 mesh and treated at a temperature of 100.degree. C. to 300.degree. C. in the presence of acid catalysts. The proportion of methanol to waste is between 1:1 by weight.
U.S. Pat. No. 4,620,032 teaches an extrusion process for reducing the reaction time in the hydrolysis of polyesters by intimately admixing with the molten polyester a depolymerizing agent which is either one of the products resulting from the complete hydrolytic depolymerization of the polyester or water.
U.S. Pat. No. 5,051,528 issued to Naujokas, et al on Sep. 24, 1991, discloses a method for recovering ethylene glycol and dimethyl terephthalate (DMT) from polyethylene terephthalate polymers (PET). The process includes the steps of dissolving scrap polyester in oligomers of ethylene glycol and terephthalate acid or dimethyl terephthalate and passing super-heated methanol through the solution. The ethylene glycol and dimethyl terephthalate are subsequently recovered overhead. The patent teaches that an oligomer of the same monomers is that of the monomers which form the constituent parts of the polymer, that is, the oligomer is formed from ethylene glycol and terephthalic acid or dimethyl terephthalate. Accordingly, the oligomer is any low molecular weight polyester polymer of the same composition as that of the scrap material being employed as the starting component so that the scrap polymer will dissolve in the low molecular weight oligomer.
U.S. Pat. No. 5,298,530 issued to Gamble, et al on Mar. 29, 1994, discloses a process of recovering components from PET having the steps of introducing glycol and terephthalic acid or dimethyl terephthalate oligomers to a first vessel and heating the oligomers, introducing scrap polyesters to the first vessel and forming a start-up melt with the oligomers, transferring the melt from the first vessel to a second vessel, passing super-heated methanol through the melt in the second vessel to form a final melt comprising low molecular weight polyesters in monomers, transferring the final melt from the second vessel to the first vessel, and recovering components in the form of a vapor stream exiting the second vessel. The process shortens the length of the polyester chain in a polyester scrap melt prior to the introduction of the scrap melt to a first reactor.
U.S. Pat. No. 5,414,022 discloses an improvement to the process of U.S. Pat. No. 5,051,528. The improvement includes the steps of adding polyester to the dissolver and combining it with melt from the reactor and liquid from the rectifier to reduce the chain length of the polyester, transferring the reduced chain length polyester from the dissolver to the reactor, passing super-heated methanol through the reactor to depolymerize polyester into its constituent monomers, transferring the depolymerized products from the reactor to the rectifier, and separating the depolymerized products in the rectifier into a vapor phase containing component monomers and a liquid phase containing higher molecular weight materials.
U.S. Pat. No. 5,576,456 issued to Gamble, et al, discloses an improvement to the process disclosed in U.S. Pat. No. 5,298,530 in that the reactor for depolymerizing the polyester into its components uses a staged column reactor for the depolymerization and for separating monomer components from the higher boiling materials. The process further utilizes a reactor in which the continuous phase is the super-heated methanol in the molten polyester and polyester decomposition products are the discontinuous phase.
The processes and equipment described in the prior art employ a reactor in which the polyester is depolymerized in the presence of superheated methanol and from which vapor is removed. The methanol stream in the reactor can be either the continuous phase or the discontinuous phase. These depolymerization processes also include a rectifier or partial condenser to return any dimer, trimer, or other oligomeric species, as well as other components with high boiling points from the vapor to the reactor.
One will understand that the above processes require the polyester scrap to be reacted with a suitable catalyst and a lower alcohol such as methanol, ethanol, propanol and the like at elevated temperatures and pressures as are known to those skilled in the polyester art. The ester of the dibasic acid, DMT, is then recovered by a rather complex and time consuming operation. Half-esters such as methyl hydroxyethyl terephthalate (MHET) are incompatible with the DMT purification process and were heretofore returned to the depolymerization reactor for further processing, otherwise the MHET is lost.
A problem with the all of the above processes for recycling polyester and particularly PET is that the recovered material must be in the form of dimethyl terephthalate (DMT) and ethylene glycol. The reason for this is that DMT, which is reacted with ethylene glycol to form diglycol terephthalate which is then polycondensed to form PET, can be purified using techniques, such as distillation and crystallization, known to those skilled in the art.
Thus, there is a widespread need for a simple and economical method that will permit the recycling of polyesters for recovering suitable feedstock material that may be used in either a terephthalic acid (TPA) based polyester manufacturing process or a DMT polyester manufacturing process. Desirably, such process may recover at least a portion of the monomer constituents from the depolymerization of the polyester.